1. Field of the Invention
The present invention relates to a method for preparing an ester from a hydroxybenzoic acid and a fatty alcohol for use in a binder system for powder injection molding, as well as a binder system for an injection molding method.
2. Description of Related Art
Binder systems, which are used in powder injection molding, generally contain a polymeric binder component, an additional binder component, a so-called co-binder, which is compatible with the polymeric binder component, and, if necessary, a dispersing agent in order to keep metal or ceramic powder particles in dispersion in the molten binder. The compatibility of the additional binder component having the polymeric binder component means that the binder components in the melt form a uniform liquid phase, and that, even in the solid state, a uniform binder phase (solid solution) is present, the further binder component having to be additionally removable from the common solid phase below its melting point, for instance, by extraction, the polymeric binder component not being permitted to form sources or cracks in a major way.
Polyamides are frequently used as polymeric binder components. As suitable co-binders for polyamides, for example, N,N′-diacetylpiperazine is known from published European patent document EP-B 0 809 556 or from U.S. Pat. No. 5,002,988, and alkylphenols from published German patent document DE-C 41 29 952. When alkylphenols are used, these are generally the ones having C1-C20 alkyl.
As further suitable co-binders for binder systems having polyamides as the polymeric binder component, a mixture of various compounds is also used, for example, which contains, among other things, esters of p-hydroxybenzoic acid with various fatty alcohols. Such a mixture was obtainable in the trade under the designation Loxiol 2472®, from the firm of Cognis, for example. However, this mixture also contains contamination, such as phenol, for example. Phenol is physiologically graded as toxic, and, at a temperature of 25° C. it already has a vapor pressure of 41 hPa. This may lead to the release of phenol during running operation, and thus to an endangerment of the health of a user. A further disadvantage of a mixture of various compounds is that a non-uniform composition may occur within a packing container. This results, among other things, from the fact that a melt filled into a barrel hardens very slowly from the outer to the inner region, whereupon fractional crystallization of the individual components occurs. This fractional crystallization has the result that the composition of the product depends on from where in the container the product is taken. The changing of the composition, depending on the place of removal, causes the reproducibility of the powder injection molding process chain to suffer in the steps feedstock processing, injection molding and solvent extraction of the co-binder. A further disadvantage is that components of the co-binder in the injection-molded component separate out. Independently of whether these separations are dissolved out during the extraction or not, a pore develops after sintering, at the location of the separation.
Especially in the use of a mixture of various compounds, which contains, among other things, esters of p-hydroxybenzoic acid with various fatty alcohols, it has also been shown that the injection-molded parts tend to stick to the injection-molding tool.
These disadvantages may particularly be attributed to the fact that fatty alcohols are obtained from natural products and generally come as a mixture. In addition, in the known synthetic method for preparing esters from a hydroxybenzoic acid with a fatty alcohol, phenol is created as a byproduct.